Electrochemical cells which employ ion exchange membranes formed of solid polymer ion exchange resins and electrodes in which catalytically-active electrically-conductive materials are included are well known in the art. Such cells can be used for the generation of electricity, for example, in fuel cells and batteries; or in electrolytic reactors, for example, for electrolysis of water, chemical synthesis, and many other uses.
Such cells are manufactured by various techniques which provide a structure of a solid polymer electrolyte (SPE) membrane, or proton exchange membrane, for ion exchange, sandwiched between electrodes for current transfer and, in the case of gaseous fuel cells, gas diffusion. Solid polymer ion exchange membranes useful in such devices can be selected from commercially available membranes, for example, perfluorinated membranes sold under the tradenames Nafion® (DuPont Co.) and Flemion® (Asahi Glass Co.); or formed as films cast from solutions containing commercially available ion exchange resins. The electrodes are often formed of electrically-conductive particulate materials (which may include catalyst materials) held together by a polymeric binder. Polytetrafluoroethylene (PTFE) resin, due to its chemical inertness and high temperature resistance, is often used as the polymeric binder. The PTFE resin is usually combined with the particulate electrode materials and molded or processed into sheet form by PTFE paste-forming processes known in the art. The cells may also include porous current collection or distribution layers, for example, platinum wire mesh or woven carbon cloth, in contact with the electrode surfaces facing away from the ion exchange membrane. Important considerations in such layered structures include uniformity in the thickness and distribution of functional materials forming, or within, the layers; and quality and durability of contact between the layers. It is also desired that the layers be as thin as possible to increase the energy efficiency and current density of the cells.
To improve the energy efficiency of the electrochemical devices, electrode structures have been modified to increase the number of reaction sites. In addition, to increase the rate of ion movement, solid polymer ion exchange resins have been included within the electrode structures. To allow the ions produced to move rapidly toward the counter electrode, it is necessary to improve the contact between the solid polymer ion exchange resin inside the electrode and the ion exchange membrane, and to lower the membrane resistance of the ion exchange membrane itself.
Conventionally, the solid polymer membranes are joined to the electrodes by hot pressing or simply held together in the cell by mechanical forces applied to them. It is difficult, however, to produce a cell with thin membranes by either method. When hot pressing is used the membrane material is softened and weakened by the heat and, if too thin, will rupture and create a gas leakage path or cause a short circuit between the electrodes. Such problems are exacerbated if the electrode surfaces have poor smoothness. When only mechanical force is used, a much greater force is required to ensure uniform contact and to obtain a low contact resistance between the membrane and electrodes, and the same problems are encountered with thin ion exchange membranes. A further disadvantage is that pressure applied to force the electrodes and membrane together, whether with or without heat, can cause compaction of the electrodes and thus reduce the gas permeability of the electrode.
Means used to address these problems include applying and drying a solution containing a solid polymer ion exchange resin to an electrode surface, and then joining the coated electrode to an ion exchange membrane by hot pressing. Another method described is to apply a solution containing a solid polymer ion exchange resin, or a solvent for the resin, to an electrode surface and then, with solvent still present on the surface, join the coated electrode to an ion exchange membrane, after which the solvent is removed. In another method a solution containing a solid polymer ion exchange resin is applied to a surface of two electrodes and, while still wet, the coated surfaces are brought together, after which the solvent is removed and an ion exchange membrane formed between the electrodes. These methods, too, suffer drawbacks in that it is very difficult to control the penetration of the applied solutions into the electrodes and excessive amounts of the solutions must be applied. This often results in impaired gas diffusitivity in the electrodes and also makes it difficult to obtain a thin ion exchange membrane with a uniform thickness.
Other methods to produce ion exchange membrane/electrode structures in which electrodes are formed on a current collector and subsequently joined to an ion exchange membrane; or in which electrodes are formed directly on an ion exchange membrane and subsequently joined to a gas diffusion material or current collector, are also known in the art. Most such methods are variants or combinations of the methods described above, except that different substrates are used, and have drawbacks similar to those described.
U.S. Pat. No. 5,234,777 (to Wilson) is for a membrane catalyst layer structure for a fuel cell which incorporates a thin catalyst layer between a solid polymer electrolyte and a porous electrode backing. Wilson discloses a catalyst film formed from an ink preparation consisting of a mixture of carbon particle-supported platinum catalyst, a solubilized ion exchange resin, and thickening agents. The electrode ink can be applied to a release surface, oven-dried to form a thin layer, and, after sufficient layers have been added to form the film, removed and hot pressed to an ion exchange membrane. An alternative method is also disclosed in which a different form of the ion exchange resin is solubilized in the ink mixture, the electrode ink is applied to the surface of an ion exchange membrane, heated and dried to form a layer, and, after sufficient layers have been added to form the film, treat the assembly to convert the ion exchange resin to its use form.